Physical Interpretation of Time‐Varying StorAge Selection Functions in a Bench‐Scale Hillslope Experiment via Geophysical Imaging of Ages of Water.

Understanding transit times (TT) and residence times (RT) distributions of water in catchments has recently received a great deal of attention in hydrologic research since it can inform about important processes relevant to the quality of water delivered by streams and landscape resilience to anthropogenic inputs. The theory of transit time distributions (TTD) is a practical framework for understanding TT of water in natural landscapes but, due to its lumped nature, it can only hint at the possible internal processes taking place in the subsurface. While allowing for the direct observation of water movement, Electrical Resistivity Imaging (ERI) can be leveraged to better understand the internal variability of water ages within the subsurface, thus enabling the investigation of the physical processes controlling the time‐variability of TTD. In this study, we estimated time‐variable TTD of a bench‐scale bare‐soil sloping soil lysimeter through the StorAge Selection (SAS) framework, a traditional lumped‐systems method, based on sampling of output tracer concentrations, as well as through an ERI SAS one, based on spatially distributed images of water ages. We compared the ERI‐based SAS results with the output‐based estimates to discuss the viability of ERI at laboratory experiments for understanding TTD. The ERI‐derived images of the internal evolution of water ages were able to elucidate the internal mechanisms driving the time‐variability of ages of water being discharged by the system, which was characterized by a delayed discharge of younger water starting at the highest storage level and continuing throughout the water table recession. Plain Language Summary: Understanding how long water stays in the earth's surface (from the moment it lands as precipitation until it reaches a stream or evaporates back to the atmosphere) is quite important for many earth science investigations. However, its estimation is still in its infancy, and not many methods exist that allow for the direct observation of water ages within the subsurface. This paper reports on an experiment that tries to do so by combining two different methods: the first method use information from chemically labeled water from influxes and outfluxes, and the second one used electrical resistivity of the soil, allowing for the imaging of the internal distribution of ages. We learned that our system tends to discharge older water from storage until it reaches its highest levels of storage, which is when greater portions of younger water start being released. This behavior can be explained by an upward movement of the water table that is able to reach the regions where younger water is located, allowing for its mobilization out of the system. Key Points: Electrical resistivity imaging was used to derive spatial distributions of water ages in a model hillslopeAnalysis of time‐varying SAS functions indicated increasing release of younger water with decreasing storageDownward and upward soil saturation mechanisms were identified as causing the observed behavior [ABSTRACT FROM AUTHOR]